Programmed cell death 1 ligand 1 (PD-L1) also known as cluster of differentiation (CD274) or B7 homolog 1 (B7-H1) is a protein that in humans is encoded by the CD274 gene and is, next to PD-L2 (CD273), one of the ligands of PD-1 (CD279). PD-L1 is a 40 kDa type 1 transmembrane protein which is expressed on many hematopoietic cells, including dendritic cells, macrophages, mesenchymal stem cells and bone marrow-derived mast cells. PD-L1 is found to be inducibly expressed on epithelial and endothelial cells through the action of interferons. Sites of immune privilege such as syncytiotrophoblats in the placenta and in the retina were found to constitutively express PD-L1.
Expression of PD-L1 in the placenta increases at the beginning of the second trimester, is upregulated by increased oxygen and is rapidly lost with low oxygen concentrations. Experiments have shown that the PD-1-PD-L pathway regulates the balance between the stimulatory and inhibitory signals needed for effective immune responses to microbes and maintenance of self-tolerance, respectively. Many microorganisms that cause chronic infection exploit the PD-1-PD-L1 pathway to evade host immune effector mechanisms. Based on a mouse model of liver infection, the PD-PD-L1 pathway is also thought to regulate immune-mediated tissue damage during viral infection, since PD-1 null mice show increased liver damage upon clearance of adenovirus compared to wild type mice, which is thought to be caused by highly active and aggressive T-cells.
Immune attack via IFNγ release leads to inducible upregulation of PD-L1 by mucosa creating an “immune shield” to protect against autoimmune attack in the setting of chronic inflammation or infection. Upregulated PD-L1 on these cells binds to PD-1 on T-cells contributing to the development of T-cell exhaustion.
Tumor cells have co-opted this PD-1-PD-L1 regulatory mechanism, which under normal physiological setting protects mucosa from autoimmune attack, and instead overexpress PD-L1 to avoid immunologic surveillance thereby promoting cancer growth.
This immunosuppressive mechanism can be hijacked by PD-L1 positive tumor cells eventually leading to the escape of tumors from the elimination by the immune system. Inhibiting the PD-1/PD-L1 interaction by means of a monoclonal antibody provides a promising concept for the treatment of tumors with PD-L1 expression. Clinical trials of blocking monoclonal antibodies against PD-1 and PD-L1 are currently ongoing for patients suffering from various malignancies.
Statistical analysis of published anti-PD-L1 clinical trial data comparing the clinical response rate of PD-L1 positive or PD-L1 negative patients indicates that PD-L1 expression is a predictive marker of clinical response for certain cancer types and a correlation biomarker for others (Gandini et al., Crit Rev Oncol Hematol. 2016 April; 100:88-98). In metastatic melanoma, for example, PD-L1 expression in tumor tissue is associated with a significant better prognosis, as PD-L1 positive patients receiving anti-PD-L1 therapy show a 53% reduction in mortality.
Studies have also shown that across multiple cancer types responses to anti-PD-L1 therapy were observed in patients with tumors expressing high levels of PD-L1, in particular when PD-L1 was expressed on tumor-infiltrating immune cells (see e.g. Herbst et al., Nature. 2014 Nov. 27; 515(7528):563-7; Ilie et al., Annals of Oncology 27: 147-153, 2016). In papillary thyroid cancer PD-L1 expression was found to correlate with recurrence and shortened disease free survival supporting the use of PD-L1 expression in this tumor type as a prognostic marker (Chowdhury et al., Oncotarget. 2016 Apr. 12).
Detection and scoring of PD-L1 expression in tumor tissue samples is usually done by means of immunohistochemistry on frozen or formalin-fixed, paraffin-embedded (FFPE) tumor tissue sections. Scoring of PD-L1 expression can done using different methodologies: One approach for example employed a binary end-point scoring of a specimen of being positive or negative for PD-L1 expression, with a positive result defined in terms of the percentage of tumor cells that exhibited histologic evidence of cell-surface membrane staining. Two different cut-off values of 1% and 5% of total tumor cells have been used at which a tumor specimen was scored as PD-L1 positive (Cancer. 2011 May 15; 117(10):2192-201; N Engl J Med 2012; 366:2443-54). PD-L1 expression in tumor specimens has also been quantified by scoring both, tumor cells and tumor-infiltrating immune cells that display a membranous staining, compared to tumor cells which showed both, membranous and a prominent cytoplasmic staining. Specimen scoring based on PD-L1 expression was subsequently done, whereby IHC scores of 0, 1, 2, or 3 were given. A specimen was scored with “0” if less than 1% of the cells were PD-L1 positive, “1” for more than 1%, but less than 5%, “2” if more than 5%, but less than 10%, or “3” if more than 10% of the cells were PD-L1 positive (Herbst et al. Nature. 2014 Nov. 27; 515(7528):563-7). PD-L1 expression in tumor-infiltrating mononuclear cells (TIMCs) has been assessed using a semi-quantitative approach according to three categories with respective scores of 0, 1, or 2 depending on the number of TIMCs in the specimen: 0%=0, <5%=1, ≥5%=2.
The specificity and reproducibility of most commercially available anti-PD-L1 antibodies has not been thoroughly assessed and limitations of some widely used antibodies have been reported (see e.g. Cancer 2011; 117: 2192-201; Carvajal-Hausorf et al., Laboratory Investigation (2015) 95, 385-396). For example, WO 2014/165422 A1, WO 2016/007235 A1 disclose an anti-PD-L1 antibodies and scoring guidelines for use with the respective antibody to assess PD-L1 expression.
There is thus a continued need to expand the repertoire in available anti-PD-L1 antibodies that are highly specific for PD-L1 and that yield reproducible results to aid in the stratification of tumor patients amenable for an anti-PD-L1-based therapy.